This invention relates to improvements in monitoring apparatus for electrical circuits, and in particular an improved apparatus for monitoring the integrity of a tripping circuit and the presence of a supply to the circuit. It is especially applicable to monitoring conditions which would prevent the energising of an un-energised electrical circuit, for example failure of a power supply for the circuit.
It is well known to provide a circuit breaker arranged to disconnect part of a high voltage (or relatively high) electrical circuit in response to the closing of a contact (the trip contact) attached to a low voltage electrical circuit. The trip contact and supply voltage form a part of a trip circuit. The circuit breaker may therefore comprise a trip coil and a circuit breaker contact. Closing the trip contact causes current to flow in the trip coil of the circuit breaker, which results in the opening of the circuit breaker and the disconnection of the high voltage circuit. When a fault occurs in the high voltage circuit, it is important that the circuit breaker can operate to disconnect the circuit, to minimise the damage caused.
It is normal practice to connect a monitoring circuit or supervision circuit across the open trip contact to provide an indication that the supply is available to energise the trip coil when the trip contact closes. In its simplest form, this circuit (hereinafter referred to as a trip circuit supervision circuit) can consist of a bulb or other visual or audible indicator connected across the trip coil as shown in FIG. 1. The bulb is lit while the trip contact is open and a supply voltage is present.
An improved monitoring circuit uses an electrical relay in place of the bulb. The contacts of this relay are held open while the trip contact remains open and the trip supply is present. Failure of the trip supply causes the relay contacts to close, initiating an audible or visual alarm.
Both these trip circuit supervision circuits present problems if the trip supply voltage varies widely.
The current flowing in the supervision circuit when the voltage is at its lowest value must be sufficient to light the bulb, or hold the relay contacts open. The current flowing in the supervision circuit when the voltage is at its highest must not be sufficient to trip the circuit breaker. The power dissipated by the supervision circuit can be excessive, at the highest voltage, particularly if the circuit is incorporated into the protective relay equipment which operates the trip contact.
It is required that failure of any component of a supervision circuit connected across a trip contact must not cause sufficient current to flow in through the circuit that will trip the circuit breaker.
An aim of at least one aspect of the present invention is to ameliorate the problems associated with the high power dissipation across the resistive element.
According to a first aspect, the invention provides a supervision circuit adapted for use in combination with a supervised circuit comprising at least one electrical component connected in series in a supply line from a supply voltage, the supervision circuit being connected across the electrical component and including a current sensing device, in which during normal operation of the supervised circuit the supervision circuit is adapted to draw a substantially constant current from the supply line independent of supply voltage fluctuations, and in which the current sensing device produces an output signal dependent upon the current drawn by the supervision circuit.
The electrical component may, for example, comprise a switch which is adapted to connect or disconnect a load in the circuit to the supply voltage.
In accordance with a second aspect, the invention provides a trip circuit supervision circuit for use in combination with a trip circuit comprising a circuit breaker trip coil connected in series in a supply line from a supply voltage and a trip relay contact connected in series with the circuit breaker trip coil in the supply line, the trip circuit supervision circuit being connected across the relay trip contact and including a current sensing device and which is, in which during normal operation the supervision circuit is adapted to draw a substantially constant current from the supply line independent of supply voltage fluctuations when the relay trip contact is open, and in which the current sensing device produces an output signal dependent upon the current drawn by the supervision circuit.
Preferably, the supervision circuit comprises a constant current source connected in series with the current sensing device.
Thus, in accordance with the invention, the prior art resistive load and bulb have been eliminated and replaced by a constant current circuit and a current sensing device. Of course, a bulb or other light emitting device could still be provided in addition as a simple check device, perhaps in series with the current sensing device.
An advantage of the invention is that the current circuit draws a current which does not vary with small changes in supply voltage as is the case of a resistive load. This results in a more stable current which is easy to monitor by the current sensing device. It also consumes less power. The power dissipated in the trip protection circuit is no longer dependent on the square of the supply voltage as with resistive load.
The output signal from the current sensing device may be used to drive a visual or audible warning device such as a bulb or siren. It could be logged by a computer or other device.
In a refinement, the constant current source may comprise first and second constant current sources connected in series with each other.
The use of the second constant current circuit acts as a safety back up in the event of failure of the first constant current source. If the first constant current circuit failed and produced a short circuit across the relay trip contact, sufficient current could flow to actuate the circuit breaker trip coil. The provision of the second constant current source prevents such a failure mode occurring by limiting the current.
The current sensing device may comprise a light emitting diode connected in series with the constant current source which illuminates a photodiode. The photodiode may be connected to a DC supply and allows current to flow when illuminated by the diode to produce the output signal. This also provides a high degree of isolation to prevent the formation of undesirable ground loops.
A switching means may be provided in series with the constant current source. By opening/closing the switching means it is possible to efficiently reduce the power consumed by the constant current source by switching the supervision circuit ON/OFF. Thus the switch may be modulated by a suitable pulse train. As an example, using a square wave would halve the power dissipation.
In accordance with a third aspect, the invention provides a trip circuit including a trip circuit supervision circuit, in which the trip circuit comprises at least one circuit breaker trip coil adapted to open a circuit breaker in a supply line in response to a current flowing in the coil in excess of a threshold value and a relay trip contact connected in series with the trip coil and in which the trip circuit supervision circuit includes a current sensing device connected in series across the relay trip contact, the supervision circuit being adapted to draw a constant current from the supply line independent of supply voltage when the trip contact is open, the current sensing device producing an output signal indicative of the current drawn by the supervision circuit.
The supervision circuit may comprise a constant current source and a current sensing device arranged In accordance with the first or second aspects of the invention.
The trip circuit may further include a circuit breaker auxiliary contact connected in series with the trip coil and the protection relay trip contact. A trip circuit supervision circuit in accordance with the first or second aspects of the invention (for example an electrical circuit including a current sensing device adapted to draw substantially constant current independent of fluctuation in the supply voltage) may be provided in parallel with the auxiliary contact. A further circuit breaker auxiliary contact may be provided across the first auxiliary contact. This further contact may be in series with the constant current circuit of the trip circuit protection circuit.